util: add utility object to track mouse position

util.mouseLocationTracker implements a subscription paradigm to conserve
processing power: it tracks mouse location in the document when at least
one resource is subscribed
mouseTrackingView: add mouseTrackingView

lacks significant programmatic testing, but implements core
functionalities in a 'looks good' visual test
slidingObjectView: add SlidingObjectView and tests
slidingBarView: add SlidingBarView and tests